1. Field of the Invention
The present invention relates to an optical image measuring apparatus employing a structure in which an object to be measured is irradiated with a light beam and a surface form or inner form of the object to be measured is measured based on a reflected light beam or a transmitted light beam to produce an image of a measured form.
2. Description of the Related Art
In recent years, attention has been given to an optical image measuring technique for producing an image of a surface or inner portion of an object to be measured using a laser light source or the like. This optical image measuring technique is not hazardous to human bodies in contrast to optical image measuring techniques using radial rays such as X-ray CT. Therefore, development of applications in particular the medical field has been expected.
An example of a typical method in the optical image measuring technique is a low coherent interference method (also called an optical coherent tomography or the like). This method uses low coherence of a broadband light source having a wide spectral width, such as a super luminescent diode (SLD). According to the method, reflection light from an object to be measured or light transmitting therethrough can be detected at superior distance resolution of μm order (for example, see Naohiro Tanno, “Kogaku”, Volume 28, No. 3, 116 (1999)).
FIG. 13 shows a fundamental structure of a conventional optical image measuring apparatus based on a Michelson interferometer, serving as an example of an apparatus using the low coherent interference method. An optical image measuring apparatus 200 includes a broadband light source 201, a mirror 202, a beam splitter 203, and a photo detector 204. An object to be measured 205 is any object such as corneal or retina of a human eye made of a scattering medium. A light beam from the broadband light source 201 is divided by the beam splitter 203 into two, that is, reference light R propagating to the mirror 202 and signal light S propagating to the object to be measured 205. The reference light R is light reflected by the beam splitter 203. The signal light S is light transmitting through the beam splitter 203.
Here, as shown in FIG. 13, a propagating direction of the signal light S is set as a z-axis and a plane orthogonal to the propagating direction of the signal light S is defined as an x-y plane. The mirror 202 is shiftable in a direction indicated by a double-headed arrow in FIG. 13 (z-scanning direction).
The reference light R is subjected to Doppler frequency shift by z-scanning when it reflects to the mirror 202. On the other hand, the signal light S is reflected on a surface of the object to be measured 205 and inner layers thereof when the object to be measured 205 is irradiated with the light. Because the object to be measured 205 is the scattering medium, reflection light of the signal light S may be a diffusing wave having random phases including multiple scattering. The signal light propagating through the object to be measured 205 and the reference light that propagates through the mirror 202 and is subjected to the frequency shift are superimposed on each other by the beam splitter 203 to produce interference light.
In the image measurement using the low coherent interference method, a difference in optical path length between the signal light S and the reference light R is within a coherent length (coherent distance) of μm order of the light source. In addition, only a component of the signal light S which has phase correlation to the reference light R interacts with the reference light R to produce interference light. That is, only a coherent signal light component of the signal light S selectively interacts with the reference light R. Based on such fundamentals, the position of the mirror 202 is shifted to change the optical path length of the reference light R, so that the interference light including information of reflection light obtained at various z-coordinates (measuring depth) of the object to be measured 205 is produced. As a result, a light reflection profile of the inner layers of the object to be measured 205 is measured. The object to be measured 205 is also scanned with the irradiated signal light S in an x-y plane direction. The interference light is detected by the photo detector 204 during scanning in such a z-direction and x-y plane direction. An electrical signal (heterodyne signal) outputted as a result obtained by the detection is analyzed to obtain a two-dimensional sectional image of the object to be measured 205 (for example, see Naohiro Tanno, “Kogaku”, Volume 28, No.3, 116 (1999)).
It is assumed that an intensity of the reference light R and an intensity of the signal light S which are superimposed by the beam splitter 203 are given by Ir and Is, respectively, and a frequency difference between the reference light R and the signal light S and a phase difference therebetween are given by fif and Δθ, respectively. In this case, a heterodyne signal as expressed by the following expression is outputted from the photo detector (for example, see Yoshizawa and Seta “Optical Heterodyne Technology (revised edition)”, New Technology Communications (2003), p.2).i(t)∝Ir+Is+2√{square root over (IrIs)} cos(2πfift+Δθ)  (1)
The third term of the right side of the expression (1) indicates an alternating current electrical signal and the frequency fif thereof is equal to a frequency of beat caused from the reference light R and the signal light S. The frequency fif of an alternating current component of the heterodyne signal is called a beat frequency or the like. The first and second terms of the right side of the expression (1) indicate direct current components of the heterodyne signal and correspond to a signal intensity of background light of interference light.
When the two-dimensional sectional image is intended to be obtained by means of the conventional low coherent interference method, it is necessary to scan the object to be measured 205 with a light beam and to successively detect reflection light waves from respective regions of the object to be measured 205 in a measuring depth direction (z-direction) and a sectional direction (x-y plane direction). Therefore, in this method, the measurement of the object to be measured 205 requires a long time. In addition, it is hard to shorten a measurement time in view of measurement fundamentals.
In views of such problems, an optical image measuring apparatus for shortening a measurement time has been proposed. FIG. 14 shows a fundamental structure of an example of such an apparatus. As shown in FIG. 14, an optical image measuring apparatus 300 includes a broadband light source 301, a mirror 302, a beam splitter 303, a two-dimensional photo sensor array 304 serving as a photo detector, and lenses 306 and 307. A light beam from the light source 301 is converted into a parallel light flux by the lenses 306 and 307 and a beam diameter thereof is increased thereby. Then, the parallel light flux is divided by the beam splitter 303 into two, that is, the reference light R and the signal light S. The reference light R is subjected to Doppler frequency shift by z-scanning of the mirror 302. On the other hand, the signal light S is incident on an object to be measured 305 over a wide area of the x-y plane because the beam diameter has been increased. Therefore, the signal light S becomes reflection light including information related to the surface and inner portion of the object to be measured 305 in the incident area. The reference light R and the signal light S are superimposed on each other by the beam splitter 303 and detected by elements (photo sensors) arranged in parallel on the two-dimensional photo sensor array 304. As described above, according to the optical image measuring apparatus 300, it is possible to obtain a two-dimensional sectional image of the object to be measured 305 in real time without light beam scanning in the x-y plane direction.
An apparatus described in K. P. Chan, M. Yamada, and H. Inaba, “Electronics Letters”, Vol. 3, 1753 (1994) has been known as such a non-scanning type optical image measuring apparatus. In the apparatus described in the same document, a plurality of heterodyne signals outputted from a two-dimensional photo sensor array are inputted to signal processing systems arranged in parallel to detect the amplitude and phase of each of the heterodyne signals.
However, when spatial resolution of an image is intended to be improved in such the non-scanning type optical image measuring apparatus, it is necessary to increase the number of elements of the array. In addition, it is necessary to prepare a signal processing system including the number of channels corresponding to the number of elements. Therefore, it is likely to be hard to actually use the apparatus in fields that require a high-resolution image, such as a medical field and an industrial field.
In view of such a problem, the following optical image measuring apparatus is proposed (see JP 2001-330558 A). The optical image measuring apparatus described in JP 2001-330558 A includes a light source for generating a light beam, an interference optical system, and a signal processing portion. In the interference optical system, the light beam emitted from the light source is divided into two, that is, signal light propagating through an examined object locating position in which an object to be examined is located and reference light propagating on an optical path different from an optical path passing through the examined object locating position. The signal light propagating through the examined object locating position and the reference light propagating on the different optical path are superimposed on each other to produce interference light. The interference optical system includes a frequency shifter, light cutoff devices, and photo sensors. The frequency shifter shifts a frequency of the signal light and a frequency of the reference light relative to each other. In order to receive the interference light in the interference optical system, the interference light is divided into two parts. The light cutoff devices periodically cut off the two divided parts of the interference light to generate two interference light pulse trains with a phase difference of 90 degrees therebetween. The photo sensors respectively receive the two interference light pulse trains. The photo sensors each have a plurality of light receiving elements which are spatially arranged and each of which separately obtains a light receiving signal. The signal processing portion combines a plurality of light receiving signals obtained by each of the photo sensors to generate signals of the signal light which correspond to respective points of interest of a surface or inner layers of the object to be examined which is located in the examined object locating position on a propagating path of the signal light.
That is, in the optical image measuring apparatus described in JP 2001-330558 A, an optical path of the interference light produced by superimposing the reference light and the signal light on each other is divided into two optical paths and the light cutoff device and the photo sensor (two-dimensional photo sensor array) are disposed on each of the two optical paths. A phase difference of π/2 is set between sampling periods of the light cutoff devices disposed on the two optical paths. Thereby, an intensity of background light of the interference light and phase quadrature components (sine component and cosine component) of the interference light are detected. In addition, the intensity of the background light is subtracted from receiving signals outputted from the photo sensors to calculate two phase quadrature components of the interference light. An amplitude of the interference light is acquired based on a result obtained by the calculation.
For example, when the optical image measuring apparatus is used to measure a medical image such as a corneal epithelial image or a retina image (eye fundus image), first, it is necessary that visible light (observation light) be emitted to an affected area and its vicinities which are to be observed to determine a region for image measurement (measurement region). Many measurement regions are expanded in a depth direction (z-direction). Upon receipt of a measurement start trigger inputted by an examiner, the optical image measuring apparatus emits measurement light from the broad band light source to the measurement region and performs the measurement while a measurement position is shifted in the z-direction to scan the measurement region. Thereby, an image of a desirable measurement region is obtained.
The measurement position is shifted in the z-direction to scan the measurement region by adjusting an optical path length of the signal light and an optical path length of the reference light relative to each other. In the above-mentioned conventional structure, the optical path length of the reference light is adjusted by moving the reflecting mirror for reflecting the reference light. On the other hand, in order to adjust the optical path length of the signal light, the structure is known in which a condenser lens for condensing a light beam from a light source, a half mirror for dividing the light beam into signal light and reference light, and a reflecting mirror for reflecting the reference light are integrally moved in an optical path direction of the signal light (see, for example, JP 2004-191114 A). When the optical path length changes, the reference light or the signal light is subjected to Doppler frequency shift. At this time, if an adjustment speed of the optical path length is not constant, the amount of frequency shift does not become constant. Therefore, in some cases, a result obtained by measurement includes false information or the measurement cannot be performed.
When the conventional optical image measuring apparatus is to be applied to the above-mentioned medical image measurement, a method of performing both observation and image measurement on an affected area and its vicinities using the same camera (such as a CCD camera) and a method of performing the observation and the image measurement using different cameras are expected.
In the case where the method using the same camera is selected, when an observation stage is shifted to a measurement stage, a time lag occurs between the input of the measurement start trigger and the acquisition of the measurable state, with the result that the measurement area cannot be preferably measured. To explain it more specifically, in the observation stage, the entire optical image measuring apparatus is moved backward and forward to adjust the focus of the camera and the affected area and the like are observed with such a focused state. When the measurement starts in this state and the optical path length starts to adjust, preferable image measurement cannot be performed until the measurable state is obtained, that is, until the adjustment speed of the optical path length becomes constant. In particular, at the time of the start of measurement, that is, at the time of determination of the measurement region, a focused region (measurement region) cannot be subjected to image measurement, so a great disadvantage that an image of a desirable region is missed occurs.
On the other hand, even in the case where the method using an observation camera and a measurement camera which are separately provided is selected, when the observation stage is shifted to the measurement stage in the conventional optical image measuring apparatus, the focus of the measurement camera cannot be speedily adjusted to that of the observation camera. Therefore, as in the case where the same camera is used, a time lag occurs between the input of the measurement start trigger and the acquisition of the measurable state. Thus, a desirable region on which focus, is achieved at the time of determination of the measurement region cannot be subjected to image measurement.